Microstructure Evolution during Accelerated Cooling Followed by Coiling of an Nb-Ti/HSLA Steel

An Nb-Ti low carbon HSLA steel with the application of Coiled Tubing steel in petroleum industry, was subjected to hot rolling followed by the subsequent cooling procedures including cooling and coiling. The effect of cooling rate and coiling temperature on final microstructure and texture was investigated via optical microscopy, SEM, and EBSD analyses. It was found that rolling at 800 degrees C results in the appearance of elongated ferrites due to the activation of the deformation-induced ferrite transformation mechanism. Cooling from the finish rolling with different rates of 2.4 to 15 degrees C/s leads to the formation of polygonal ferrite and retained austenite. Higher cooling rate increases the retained austenite, decreases the ferrite grain size and improves the grain size homogeneity. Coiling at 600 degrees C results in coarser ferrite with the texture of gamma fiber which is due to the recrystallization of ferrite. On the other hand, coiling at 500 degrees C intensifies the {332} < 113 > and {113} < 110 > components indicating deformed austenite transformed to ferrite.